mysql查找删除重复数据并只保留一条实例详解

有这样一张表，表数据及结果如下：

school_id	school_name	total_student	test_takers
1239	Abraham Lincoln High School	55	50
1240	Abraham Lincoln High School	70	35
1241	Acalanes High School	120	89
1242	Academy Of The Canyons	30	30
1243	Agoura High School	89	40
1244	Agoura High School	100	50

我们可以看出，school_name的字段值有重复数据(Abraham Lincoln High School 和Agoura High School分别出现两次)，那么如何删除这两条数据，从而只让这两个数值出现一次呢？ 具体实现方法如下：

1、删除重复记录,保存Id最小的一条

delete FROM `test` WHERE `school_name` in (SELECT `school_name`
FROM `test` 
GROUP BY `school_name` 
HAVING COUNT( * ) >1) and school_id not in (select min(school_id) from test group by school_id having count(* )>1)

先使用GROUP BY having语法查询出重复的数据，然后删除重复数据并保留school_id最小的一条.

2、删除重复记录,保存Id最大的一条

delete FROM `test` WHERE `school_name` in (SELECT `school_name`
FROM `test` 
GROUP BY `school_name` 
HAVING COUNT( * ) >1) and school_id not in (select max(school_id) from test group by school_id having count(* )>1)

原理和上面一样。

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